Loss of synchronization during communications in a digital communication system such as a second generation cordless telephone system (CT2) creates unwanted problems to system users. Loss of synchronization can be induced by selective multipath fading, flat fading, weak signals, or other well known communication phenomena.
In a CT2 system, a handset (also known as a cordless portable part or CPP) initiating a call to a base station (cordless fixed part or CFP) asynchronously transmits on one available channel of the handset's transceiver, which corresponds to a radio frequency (RF) channel in the base station. In a typical CT2 communication system, the communication protocol standard includes four main burst structures, called multiplex 3 (MUX 3), multiplex 2 (MUX 2) and multiplex 1 (MUX 1) which is further subdivided into either multiplex 1.4 or 1.2 (MUX 1.4 or MUX 1.2). MUX 3 is utilized mainly for communication link initiation (link establishment and re-establishment) from a CPP to a CFP. MUX 2 is used primarily for communication link establishment and for link initiation from the base station (CFP). The MUX 1 burst structures (MUX 1.2 and MUX 1.4) are used primarily for voice/data communications, signaling information, and control messages from the CPP (portable) and CFP (base).
Both MUX 3 and MUX 2 contain synchronization patterns which provide frame alignment in both the CFP and CPP. MUX 1 does not include a synchronization pattern and requires precise tracking of the receive data transitions in order to maintain frame synchronization.
For a better understanding of the overall CT2 system communication protocol, one is referred to a publication entitled, "Common air interface specification to be used for the interworking between cordless telephone apparatus in the frequency band 864.1 MHz to 868.1 MHz, including public access services", Version 1.1, dated 30th June 1991, which is published by the European Telecommunications Standards Institute and is hereby incorporated by reference.
In today's digital systems, especially in systems which provide no synchronization patterns, there is a need for a PLL which can handle both clock recovery of the received data and can withstand weak signals, interference, multi-path fading conditions, or other well known problems.